1. Field of the Invention
The present invention relates to a driving force transmission device that can switch between transmission and disconnection of a rotational driving force between a first shaft body and a second shaft body.
2. Description of Related Art
A rotation transmission device described in Japanese Patent Application Publication No. 2013-92191 (JP 2013-92191 A) includes an electromagnetic clutch, and a roller clutch as a two-way clutch. The roller clutch is configured such that a pair of opposed rollers, and a spring that urges the rollers in directions away from each other are placed in each of wedge spaces formed by an inner periphery of an outer ring and an outer periphery of an inner ring. The pair of rollers is supported by two cages, that is, a control cage and a rotational cage. A ball cam structure that can rotate the control cage and the rotational cage relative to each other along with an axial movement of the control cage is provided between facing surfaces of a flange of the control cage and a flange of the rotational cage. By axially moving the control cage by current application/power-off of the electromagnetic clutch, the control cage and the rotational cage are rotated relative to each other. Thus, the pair of rollers is changed between an engaged state and a disengaged state between the inner periphery of the outer ring and the outer periphery of the inner ring.
However, in the rotation transmission device described in JP 2013-92191 A, the ball cam structure is integrated with the rotational cage. This complicates a shape of the rotational cage, which causes an increase in cost. Further, heat treatment or the like is necessary because a slide-contact surface between a ball and a cam is required to have a high contact pressure property (a high hardness property). This also causes an increase in cost. The cam can be formed by use of ceramic rather than a steel material, but this also leads an increase in cost. Accordingly, as a mechanism to switch between a fastened state and a released state of the roller clutch, it is desired to employ a new structure with which the ball cam structure can be replaced.
The inventors of the present invention have considered employing a two-way clutch including first and second cages provided in a rotatable manner relative to each other and a plurality of guide members, as a two-way clutch included in a driving force transmission device. The guide member has a first slide-contact surface constituted by an inclined surface. Further, the guide member has a second slide-contact surface constituted by an inclined surface. Along with a movement of the guide member toward an axial first side, the first cage rotates toward a circumferential second side, so as to press one of the rollers toward the circumferential second side. Further, along with a movement of the guide member toward the axial first side, the second cage rotates toward a circumferential first side, so as to press the other one of the rollers toward the circumferential first side. By pressing the rollers as described above, the rollers can be separated from an engaged position. The first and second slide-contact surfaces are set such that respective rotation amounts of the first and second cages along with the axial movement of the guide member are equal to each other.
However, the following situation may be caused for some reasons: one of the first and second cages rotates smoothly, but the other one of the first and second cages has difficulty in rotating (a state may occur in which the other one of them does not rotate or has difficulty in rotating). In this case, when the guide member moves toward the axial first side due to the drive of the electromagnetic clutch, the guide member itself also moves in a circumferential direction. As a result, the one of the first and second cages may rotate excessively by a rotation amount more than an expected rotation amount. It is desirable that excessive rotation of the one of the first and second cages be prevented even in a case where the other one of the first and second cages has difficulty in rotating.
The inventors of the present invention have considered employing a two-way clutch including first and second cages provided in a rotatable manner relative to each other, a guide member, and an electromagnetic clutch, as a two-way clutch included in a driving force transmission device. The electromagnetic clutch is connected to the guide member, and includes an armature axially movable, and a rotor axially opposed to the armature. In a fastened state of the two-way clutch, the electromagnetic clutch is in an OFF state. At this time, the guide member is placed at an initial position. In the meantime, in a released state of the two-way clutch, the electromagnetic clutch is in an ON state. When the electromagnetic clutch is turned into an ON state, the armature is attracted by the electromagnetic clutch toward the axial first side. In association with this, the guide member is drawn toward the axial first side so as to be placed at a drawn position.
Along with a movement of the guide member toward the axial first side, the first cage rotates toward the circumferential second side, so as to press one of the rollers toward the circumferential second side. Further, along with a movement of the guide member toward the axial first side, the second cage rotates toward the circumferential first side, so as to press the other one of the rollers toward the circumferential first side. In the meantime, a magnitude of a drawing force (a drawing at which the electromagnetic clutch draws the guide member) of the electromagnetic clutch is inversely proportional to the square of a distance between the armature and the rotor. That is, a drawing force provided by the electromagnetic clutch is small at an initial stage of the drawing of the guide member by the electromagnetic clutch. Accordingly, it is desired to increase a running torque applied to the first and second cages at the initial stage of the drawing of the guide member by the electromagnetic clutch.
The inventors of the present invention have considered employing a two-way clutch including first and second cages provided in a rotatable manner relative to each other and a guide member, as a two-way clutch included in a driving force transmission device. The guide member makes contact with the first cage from the circumferential first side. Further, the guide member makes contact with the second cage from the circumferential second side. Along with a movement of the guide member toward an axial first side, the first cage rotates toward the circumferential second side and the second cage rotates toward the circumferential first side. Due to the rotation of the first and second cages, roller pairs placed between an inner periphery of an outer ring and an outer periphery of an inner ring can be moved, thereby making it possible to change the state of the two-way clutch from an engaged state to a disengaged state. Respective rotation amounts of the first and second cages along with the axial movement of the guide member are set to be equal to each other.
However, the following situation may occur for some reasons: the first or second cage has difficulty in rotating (a state may occur in which the first or second cage does not rotate at all or has difficulty in rotating). In a case where one of the cages has difficulty in rotating and the other one of the cages smoothly rotates without any problems, when the guide member moves toward the axial first side due to the drive of the electromagnetic clutch, the rotation of the cage (the first cage in the above example) that does not have difficulty in rotating may rotate excessively by a rotation amount more than an expected rotation amount.
In order to drive the roller clutch including the roller pairs, that is, in order to drive the roller pairs in the roller clutch, it is necessary to rotate both of the first and second cages successfully. For this purpose, it is desirable not to allow excessive rotation of at least one of the first and second cages.